This function converts an input continuous domain rational transfer function (H = b_in/a_in) to its corresponding
z-domain digital transfer function. The function is intended to be used with normalized, s-domain, lowpass
filter functions as can be obtained from functions such as: fnbut
, fncheb1
, fncheb2
, fnbes
.

The sampling interval for the new filter is "dt", making the system sampling rate fs = 1/dt. Generally, the new
desired cutoff frequency is in [radians/sec], wc = 2*PI*fc, and should be strictly less than half of the sampling rate.

The complex gain for this resulting filter can be evaluated with the gainz
function.

The output z-domain polynomial "bz" and "az" must be declared before calling the function, though their types do not matter.